The diagnosis of human ailments by the medical profession has been significantly upgraded by the new and improved equipment for imaging the interior of patients. In recent times imaging systems that do not use dangerous radiant energy such as magnetic resonance imaging systems (MRI) have come into vogue. Such systems significantly reduce the danger of exposure to radiation and nonetheless provide images of patients that are at least on a par with images acquired using x-rays.
One of the methods for obtaining magnetic resonance (MR) images employs superconducting magnets to provide the required high static magnetic fields. The superconducting magnets use cryostats to provide the low temperatures necessary for the supeconducting condition. The cryostats presently used to obtain whole body images are for the most part horizontal cylinders with room temperature bores that are large enough to receive the body of the patient. The low temperature is maintained using liquified gases such as helium. Access to the container or tank for the liquified gases is obtained through a vertical turret attached to the horizontally oriented cylindrical cryostat. The helium tank includes the superconducting magnet itself in a "bath" of liquid helium. It is usual to use a container of liquid nitrogen juxtaposed to the helium tank to reduce helium boil-off. Both the nitrogen and helium containers are in a vacuum vessel and the containers are cylindrical units with circular cross sections co-axial with and surrounding the bore.
The turret basically comprises a "neck" tube which in general is a large diameter thin wall vertical tube typically of a diameter of approximately 4 inches made of stainless steel. The neck tube is connected by means such as welding to the helium container. A neck tube flange is connected by means such as welding near the top of the tube.
In the prior art, the neck tube flange is rigidly mounted to the walls of the vacuum vessel. The neck tube and the flange of the prior art form a vacuum boundary as part of the vacuum vessel.
An inverted can referred to in general as a "top hat" is connected by means such as bolts to the top of the neck tube flange. The top of the "top hat" contains ports for such things as the superconducting magnet current leads as well as for providing access to the liquid helium and liquid level guages, temperature sensors and safety means such as pressure release devices.
In order to minimize the conductive heat load of the helium container, it is not rigidly coupled to the cryostat. The helium container, is thus, able to move, albeit slightly in axial, rotational and transverse directions. In addition the radius of the helium container shrinks by several millimeters when it is cooled to the temperature of the liquid helium from room temperature. These movements require flexibility in the neck tube to keep it from being over stressed and even actually breaking thereby rupturing the vacuum vessel and providing a path for helium to get into the vacuum vessel.
In the prior art a metal bellows is often included integral to the neck tubes. Such prior art neck tubes comprise a metal bellows between straight cylindrical sections. The connection between the straight cylindrical sections and the bellows must of course be leak tight because any leak would introduce the helium to the vacuum space of the cryostat. Even using the integral bellows in the neck tube, the tube and bellows assembly still is not sufficiently movable to enable the rotation of the container about its own axis. Even a rotation of less than a degree overstresses the neck. Such a rotation could be induced by non-uniform strain during cool down or be caused by the presence of ferro-magnetic material near the cryostat when the magnet is energized. The problem of preventing stress cannot be solved by mounting the helium container more rigidly in order to prevent the stress causing movement because such mounting increases helium boil off and thereby increases the cost of operating the cryostat.
Accordingly there is a present and long standing need for providing neck tubes of the turrets that are sufficiently supple to move in different directions without inducing helium boil off and/or decreasing the life of the cryostat.